Example of a water tank intended for watercraft tests or inspections is known from Japanese Patent Laid-open Publication No. HEI-11-132898 disclosing a circular water flow tank and water-flow-rate adjusting method. The disclosed circular water flow tank includes an open water passage having its front and rear ends connected in fluid communication with each other via a closed water passage, and a water circulating pump disposed in the closed water passage. The disclosed circular water flow tank also includes a water-flow-rate varying means disposed in a boundary opening between the open and closed water passages, and the water-flow-rate varying means is operable to increase or decrease the height of the boundary opening and thereby control the depth of water flowing from the closed water passage to the open water passage. By the water-flow-rate varying means thus controlling the water depth, the circular water flow tank can avoid occurrence of undesired hydraulic jumps and perform appropriate water flow rate adjustment with very smooth water surface variation. Therefore, the disclosed circular water flow tank can be used suitably for testing or inspecting a watercraft.
However, the disclosed circular water flow tank, where the water circulating pump is provided in the closed water passage, is unsatisfactory in that, when the propulsion of the watercraft is great, the pump would become significant resistance to the water flow in the closed water passage, so that a proportion of the water tends to stagnate between the rear of the watercraft and the pump. When a great amount of the water stagnates between the rear of the watercraft and the pump, the performance test of the watercraft could not be performed under generally the same conditions as actual use (operating) conditions of the watercraft. If the closed water passage and water circulating pump are increased in size with a view to avoiding the inconvenience, the overall cost of the circular water flow tank would increase.